The present invention is directed to a scanning lens system for a scanning particle beam apparatus which has deflection elements and a lens system for corpuscular radiation.
When an electron probe or an ion probe is scanned over a region of a subject with an electrical or with a magnetic deflection system, the probe is usually smeared proportional to the deflection and proportional to the energy width of the beam to form a deflection aberration figure. In addition to the deflection coma, which is not easily connectible, this deflection chromatic aberration significantly contributes to the limitation of the scanned field in scanning particle beam machines for masks and wafers.
Various proposals for the correction of the deflection chromatic aberrations by means of combined electrical and magnetic deflection have been made. For example, a deflection system has been proposed for this purpose which differs from a WIEN filter only on the basis of the selection of the relationship of electrical field strength E to magnetic field strength B. In a WIEN filter, the ratio E/B is selected identical to the mean particle speed v so that the deflection disappears for particles whose speed coincides with the beam particle speed v. However, the dispersion, which causes a deflection chromatic aberration, remains finite. It has been proposed to make the ratio E/B equal to half the mean particle speed, i.e., equal to v/2. Although the dispersion will disappear, the deflection does not disappear for a particle whose speed is equal to the mean particle speed v given this selection of the ratio E/B.
A proposal is disclosed in U.S. Pat. No. 4,335,309 for the correction of the deflection chromatic aberration by means of a combined electrical and magnetic deflection. When given employment of a series-connected electrical and magnetic deflection, the purely magnetic deflection is selected to be twice as great and oppositely equal to the purely electrical deflection and then the deflection chromatic aberration disappears for small deflection angles. Given this principle of series-connected electrical and magnetic deflection, the deflection system must be disposed following the last imaging lens in order to retain the correction of the deflection chromatic aberration in the particle beam path.
An article, Hans Pfeiffer, "Recent Advances in Electron-Beam Lithography for the High-Volume Production of VLSI Devices", IEEE Transactions on Electron Devices, Vol. ED-26, No. 4, April 1979, pp. 663-673, discloses an arrangement of a magnetic deflection element in the field of a magnetic projection lens for the correction of the deflection chromatic aberration. Since, however, only an isotropic but not an anistropic component of the deflection chromatic aberration can be corrected by the arrangement of a single magnetic deflection element in the field of a magnetic projection lens, the second magnetic deflection element rotated relative to the first magnetic deflection element is proposed in the field of this magnetic projection lens.